Effects of thermal time on crop physiology and performance

In terms of the most important commercial parameter, fresh tuber weight, the three cultivars of taewa studied show resistant traits over the range of thermal time studied. Reust et.al (2001) use the term "rusticity" to distinguish cultivars that display resistance to the effects of aging. These rustic potatoes tend to show resistant traits in terms of physiological aging and can accumulate more heat units without adversely affecting crop yields when compared to modern commercial potatoes (Reust et al., 2001). Before the advent of widespread cool storage seed potatoes were kept at ambient temperatures above 4°C. This would have led to the selection of tubers that could be exposed to more thermal time while maintaining vigour. As a consequence they would have contributed more seed tuber to the next crop.

Struik et al. (2007) recommend that to get a full picture of the effect on physiological aging in a tuber that it should be de-sprouted before being planted. This is due to the sprouts themselves undergoing an aging process as they progress and as such, experiments where the sprouts are left on are technically a measure of the effects of aging on both the tuber and its sprouts (ibid). It is not common in large commercial practices to de-sprout tubers before planting. This experiment looked to mimic the current commercial cropping situation in New Zealand and did not de-sprout tubers before planting.

It is interesting to note that it is standard practice for some Māori gardeners to completely de-sprout seed tubers before planting (Roskruge, Pers. Comm., October

5, 2012). It is known that in some cultivars de-sprouting physiologically older seed can improve its performance (Struik and Wiersema, 1999). Traditionally taewa growers did not have access to modern cool storing facilities. It would seem this practice was adopted in Māori gardens in order to improve the performance of seed stored at ambient temperatures.

7.4.1 Effects on crop growth.

The hypothesis applied was that aging seed tubers could increase the yield of short season taewa crops by accelerating crop development, leading to an increased early harvest over that of younger seed tubers. Aging seed tubers can shift the growth curve used to describe the accumulation of dry matter in tubers to the left, while also decreasing the potential maximum yield (Figure 4.1). As such, emergence and time to inflorescence were measured in order to see if thermal had any effect on the rate of development of the crop. There was no effect on either emergence or the timing to inflorescence over the range of thermal time studied. It is difficult to say whether these parameters are useful measures of the age of the seed tubers as there seemed to be little overall effect of thermal time on the three cultivars

studied. Future studies could assess the rate of canopy and tuber development over a wider range of aging to help qualify the effects of aging in taewa.

7.4.2 Stem number.

In Kowiniwini there was no significant difference in terms of the effect of dd on stem number. In Moemoe, increasing thermal time from 448 to 1008 dd significantly increased stem number. As mentioned earlier, an increased number of stems results in an increased number of tubers. The 1008 dd group in Moemoe did have more

tubers (see section 7.4.4.). Waiporoporo in contrast had a decrease in the number of above ground stems with increased thermal time. This is possibly a result of

variation in tuber size as the tubers for the 448 dd group were slightly but

significantly larger than the other two treatment groups. This is explored in section 7.6.

It is interesting to note that even though the 448dd Moemoe seed tubers had more sprouts than the 1008dd seed tubers, the number of stems in the 1008dd plants was greater than the 448dd plants. There is a general trend for an increased number of sprouts on seed tubers to increase the number of stems in potato plants (Genet 1985; Gill et al., 1989). The stem count was done at removal from cool-store, not before planting. It is possible that on the day of planting the 1008 dd Moemoe seed tubers had more sprouts than the 448 dd Moemoe seed tubers. Other factors such as stem dominance as well as soil and environmental factors affect the emergence of stems (Genet, 1985; Mahmood and Gill, 1984; Gill et al., 1989). Further work would be needed to understand the influence of sprout number and other factors on the number of emerged stems.

7.4.3 Effect of Thermal Time on harvest parameters.

Whilst there was an apparent trend for decreased tuber fresh weight with increased thermal time in Moemoe at 60 DAP this was not statistically significant. There was no effect of dd on any parameter measured at the 60 DAP harvest. There was a significant trend at 90 DAP for an increase in tuber number with increased thermal time in the cultivar Moemoe. This effect is consistent with other findings in the

contrast with Waiporoporo where there was no effect, highlighting that the effect of aging on crop performance is cultivar specific. There was a trend for Moemoe to have increased tuber fresh weight at the 90 DAP harvest, but this was not significant due to the large variations in tuber size, which is a typical taewa trait (Roskruge, 1999; Hayward, 2002)

Waiporoporo showed a statistically significant increase in the dry matter

accumulated into tubers in the 728 dd treatment group. It is known that younger seed tubers can accumulate more tuber dry matter than older seed tubers (Caldiz et al. 1996). The 448 dd and 1008 dd groups however accumulated the same amount of tuber dry matter. There may be an optimal degree of aging for Waiporoporo in terms of tuber dry matter, and this would have to be investigated further before any firm conclusions could be reached.

Increased in tuber number led to a trend in decreased average tuber weight, but this was not significant. The effect of thermal time on the number of tubers set has implications for producers. Those who wish to produce Moemoe seed tubers for example, may want to leave their seed tubers to age longer than those wishing to produce for the ware or process markets. The manipulation of storage regimes in this way can help optimize the producers output for their target market (Struik and Wiersema, 1999; Hagman, 2012).

Conversely, the lack of any significant effect on Waiporoporo tuber fresh weight suggests that producers wishing to grow this cultivar can possibly save on the cost of

cool storing seed by bringing tubers out of cool storage earlier without fear of impacting crop yield. However, there does appear to be an effect on tuber dry matter that would need to be investigated further before any firm recommendations could be given. Whilst the same could be said for Moemoe and Kowiniwini, there are trends that suggest there could be an effect on tuber yields, and further work would be needed to confirm these trends.

The tubers experienced approximately 2089 dd before the treatments began

(Appendix 1). Despite the many factors that can influence physiological age the total thermal time experienced by tubers from haulm killing to planting is a good measure of the degree of aging (Caldiz, 2009). The three cultivars studied were raised,

harvested and stored at the same time and in the same manner. Therefore only the difference in thermal time after storage was considered.

The total thermal time ranged from approximately 2537 dd to 3097 dd. This exceeds the thermal time some studies with modern commercial cultivars have used where there have been detrimental effects on crop harvests (O'Brien et al., 1983; Allen and O’Brien, 1986; Asiedu et al., 2003; Struik et al., 2006.) This suggests that taewa have resistance to the effects of thermal time on physiological aging when compared to modern cultivars.